An electret condenser microphone unit included in an electret condenser microphone has a high output impedance. Accordingly, an impedance converter mainly composed of FETs is disposed between the condenser microphone unit and its output terminal.
The impedance converter is coupled to, for example, a fixed pole side of the electret condenser microphone unit. In this case, a diaphragm that is oppositely disposed with a gap to the fixed pole is grounded. An operation power source fed to such an electret condenser microphone generates a potential difference between the diaphragm and the fixed pole. In the electret condenser microphone, an electret layer then generates an equivalent polarization voltage between the diaphragm and the fixed pole. The potential difference between the diaphragm and the fixed pole adversely affects the equivalent polarization voltage.
For example, if the potential difference between the diaphragm and the fixed pole increases the surface potential of the electret layer, the electrostatic attractive force increases between the diaphragm and the fixed pole. If the electrostatic attractive force between the diaphragm and the fixed pole is too large, the diaphragm is pulled to and contacts the fixed pole, which causes the diaphragm not to vibrate and thus to preclude the function as a microphone. That is, an electric potential that increases the surface potential of the electret layer leads to a failure of the electret condenser microphone unit.
Conversely, if the potential difference between the diaphragm and the fixed pole decreases the surface potential of the electret layer, while the diaphragm does not contact the fixed pole, the electric signal output from the fixed pole side in response to the vibration of the diaphragm is low. That is, the sensitivity of the electret condenser microphone unit is reduced. In order to prevent such a failure and a reduction in the sensitivity, no potential difference should not be generated between the diaphragm and the fixed pole included in the electret condenser microphone even under a condition in which the operation power source is fed.
A phantom power source is known as an operating power source for the electret condenser microphone. The phantom power source supplies the electret condenser microphone with, for example, 48-V power via a supply resistor. The electret condenser microphone having the phantom power supply includes a three-pin connector. One of the three pins is a ground terminal. The other two pins, called a hot terminal and a cold terminal, are output terminals from which sound signals are balance output.
FIGS. 2 and 3 are circuit diagrams showing a typical conventional electret condenser microphone. The voltage added to gate terminals of FETs included in an impedance converter 200 of the electret condenser microphone shown in FIGS. 2 and 3 is half of the voltage fed from a secondary center tap of an output transformer 300. Assuming that the current consumption of the microphone is 3 mA for a voltage of 48V from the phantom power supply, the potentials of a PIN 2 (a hot terminal) and a PIN 3 (a cold terminal) are about 38 V. The potentials of the gate terminals of the FETs included in the impedance converter 200 are about 19 V.
Assuming that the equivalent polarization voltage between the diaphragm and the fixed pole included in the electret condenser microphone unit 100 is −20 V, the potential (19 V) of the gate terminal of the FET and the equivalent polarization voltage are cancelled. This reduces the equivalent polarization voltage in the electret condenser microphone unit 100 to substantially about 1 V, in other words, reduces the sensitivity of the electret condenser microphone unit 100.
A conventional method for solving the above-mentioned problems includes a bridge circuit 400 as shown in FIG. 2. The bridge circuit 400 includes four resistors, i.e., a first resistor 401, a second resistor 402, a third resistor 403, and a fourth resistor 404. The diaphragm side of the electret condenser microphone unit 100 is coupled to a node between the first resistor 401 and the second resistor 402. The output side of the impedance converter 200 is coupled to a node between the third resistor 403 and the fourth resistor 404. The bridge circuit 400 in such a circuit configuration can eliminate the potential difference between the diaphragm and the fixed pole in the electret condenser microphone unit 100. The elimination of the potential difference between the diaphragm and the fixed pole in the electret condenser microphone unit 100 can prevent a failure and a reduction in the sensitivity of the electret condenser microphone unit 100.
As shown in FIG. 3, two capacitors 201 disposed between a fixed pole of an electret condenser microphone unit 100 and an impedance converter 200 enables the DC potential to be zero in the fixed pole of the electret condenser microphone unit 100. Since the diaphragm side is grounded, the potential difference between the fixed pole and a diaphragm can be eliminated, and thereby preventing an adverse effect on the equivalent polarization voltage.
While the electret condenser microphones shown in FIGS. 2 and 3 each include an output transformer 300 in the output circuit, an electret condenser microphone without the output transformer 300 in the output circuit is also known. An output circuit with no transformer includes an emitter follower circuit as the output circuit of the electret condenser microphone (refer to PTL 1, Japanese Unexamined Patent Application Publication No. 2012-175129).
Like the electret condenser microphone shown in PTL 1, the electret condenser microphone with a transformerless output circuit has a simple circuit configuration. In an electret condenser microphone with a transformerless output circuit, the potential between a hot terminal and a cold terminal that are output terminals and the potential at a gate terminal of an FET included in an impedance converter are very close. If the consumption current is 3 mA for 48 V fed to the electret condenser microphone by phantom power feeding, the potential between the hot terminal and the cold terminal is about 38 V. In this case, the potential at the gate terminal of the FET included in the impedance converter is about 37 V.
Assuming that the equivalent polarization voltage between the diaphragm and the fixed pole is −20 V, the polarity of the potential (37 V) at the gate terminal and that of the equivalent polarization voltage is inverted, thereby causing the contact of the diaphragm to the fixed pole. If a transformerless output circuit is used, the potential difference between the diaphragm and the fixed pole is high, thereby reducing the sensitivity of the microphone.
In an electret condenser microphone, a potential difference between a diaphragm and a fixed pole by an operating power source causes a failure and a reduction in the sensitivity regardless whether a transformer is included in an output circuit or not. The potential difference between the diaphragm and the fixed pole in the electret condenser microphone including a transformerless output circuit can be zero in the configuration shown in FIGS. 2 and 3. However, the circuit configurations shown in FIGS. 2 and 3 are complicated. It is therefore desirable to reduce the potential difference between the diaphragm and the fixed pole to zero without a complicated circuit configuration.